System for determining target positions in the body observed in CT image data

ABSTRACT

This invention relates to the quantitative determination of a target or volume in the living body relative to external apparatus based on CT (computer tomography) or other image data. The invention describes a means of taking CT image data of the anatomy and of natural or implanted index positions on the patient&#39;s body, and relating the CT data to a coordinate system related to external apparatus which may be in fixed position relative to the patient. One implementation uses a transformation between the CT image data coordinate system and the second coordinate system. This makes it possible to do frame-based or frameless stereotactic target identification and approaches without the need for attaching a frame to the patient at the time of CT scanning.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/969,078,filed Nov. 12, 1997, which is now U.S. Pat. No. 6,122,341, issued Sep.19, 2000, which is a continuation of application Ser. No. 08/711,348filed Sep. 5, 1996, now abandoned, which is a continuation ofapplication Ser. No. 08/434,246 filed May 3, 1995, now abandoned, whichis a continuation of application Ser. No. 08/271,232, filed Jul. 6,1994, now abandoned, which is a continuation of application Ser. No.07/897,873, filed Jun. 12, 1992, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

The use of CT (computer tomography) imaging methods in medicine is nowwidespread. It is also commonplace to attach frames or mechanicaldevices to the patient during CT scanning. A common application is inbrain surgery where a head ring is attached to the patient's skull forthe purpose of providing a reference platform at the time of CT imagescanning. Typically, the head ring is fastened directly to the patient'skull by head posts and skull fixation means, such as sharpened, pointedscrews on the head posts that anchor directly to the skull. This framemay then be used as a rigid platform onto which may be attached alocalizer structure. When a patient is scanned with the localizerstructure so rigidly attached to his skull, index marks from thelocalizer will appear on the scan slices and provide means formathematically determining the coordinates of every image point seen inthe CT slice relative to the head frame. This technique was described indetail in U.S. Pat. No. 4,608,977 by R. A. Brown, patent issued Sep. 2,1986. This methodology has been commercialized successfully byRadionics, Inc. in the form of the BRW Brown-Roberts-Wells StereotacticInstrument. The method has been successfully used for X-ray-based, CT,MRI, and P.E.T. scanning computer tomographic scanning.

A difficulty with this technique is that it requires placement of thehead frame to the patient's head prior to CT scanning, and, in thesurgical phase, the head ring must remain on the patient's head betweenthe time of scanning and the time of surgery. This is satisfactory forsurgery such as brain biopsy when the stereotactic biopsy surgery willbe done within hours of the CT imaging. Typically it will involveputting the head frame on, attaching the localizer system to the headframe, performing the CT image scan to determine the index fiducialpoints of the localizer structure on the CT image planes, doing acalculation to relate mathematically the image data via the fiducialpoints to the coordinate frame of the head ring, and attaching astereotactic arc system or guidance means to the head frame so as toachieve the calculated target with an instrument or probe, and thus, todo the biopsy. A similar methodology is practiced for so-calledradiosurgery in which a similar procedure is carried out, except thatthe surgical probe is actually a beam of photons, and the guidance meansis an accelerator which provides the source of photons. These photonsare beamed to the target for treatment or destruction of the targetvolume by attaching the head ring directly to the accelerator and movingthe head ring in space so that the target volume stands at theconfluence of the photon beam paths.

It would be of significant advantage to be able to carry out the CTimage data without having to place the head frame on the patient's head,rather than to have the head frame remain on the patient's head betweenthe time of scanning and the time of surgery or radiosurgery. Forinstance, it may be wished to analyze the CT scanning data over anextended period so as to plan for a complex surgery or radiosurgery. Itmay also be desired to do repeat surgery or radiosurgery many timesafter image scanning, and extended over periods of days or weeks, makingit inconvenient or impractical to leave the head ring attached to thepatient's head for that period. Thus, it would be desirable to performthe CT image scanning at one time, and to be able to relate the imagescanning data at any time in the future to a head frame or fixationmeans which may be applied to the patient at that later time of surgeryor scanning. For example, a surgical head holder may be applied to thepatient's head at the time of surgery after the patient has beenanesthetized. It would be desirable to be able to relate the position ofthat head frame to the volumetric image data that has been acquired fromthe CT imaging procedure at a previous or at a subsequent time.Furthermore, it may be desirable not to attach the head frame to thepatient at any time, but rather use so-called frameless stereotacticmeans, such as an operating arm, or other device, to accessquantitatively volumes of targets in the patient's head based on the CTimage data which has been stored and manipulated in a computer.

Thus, one specific implementation of the present invention relates to amethod of achieving the above-state goal of being able to perform CTimaging without a head frame, accumulate that data in a computer,subsequently have the patient in the operating setting or radiosurgeryor radiotherapy setting and, at that time, relate the CT image data to acoordinate reference frame in the operating theater or radiation suite,such as, for example, a head frame put on at the time of surgery. Nohead frame would need to be attached during the CT imaging or at thetime of surgery necessarily, however, in one embodiment of the presentsystem, a head holder may be applied at the time of surgery or radiationsurgery, and the coordinate system related to that head holder ismathematically related to the coordinate system of the CT scanningmachine by means of X-ray or optical imaging carried out at the time ofsurgery. In one embodiment, index markers may be placed on the patient'sskin or attached to the patient's skull as an example of a means ofdefining a specific coordinate system related to the CT imaging itself.At the time of surgery, if a head frame is attached, X-ray images of thepatient's head may be taken, and the index markers that have beenattached to the patient's head may be visualized relative to index orfiducial points attached to the head frame. By analyzing the relativeposition of the patient-attached index points to the head frame-attachedindex points, a transformation can be made to relate the respective CTimage coordinate frame to the coordinate frame of the head frame, thelatter being the surgical or radiosurgical coordinate frame. Thus, inone embodiment, the present invention relates to a transformation fromCT image coordinate frame to surgical coordinate frame at the time, orapproximately the time, of surgery. The surgeon is thereby freed fromthe need to place a head frame onto the patient at the time of imaging,and subsequent surgical episodes can be done repeatedly with veryprecise reproducible means of mapping the CT image field onto thesurgical field.

An alternate example of the present invention is to place the indexmarks on the patient's skin or to the patient's bone anatomy, such asthe skull, using a frame or guidance means so as to put the markers in aspecific, relative orientation. One example would be to put them on theprincipal axes of a Cartesian coordinate system relative to the skull.This could easily be done by an appropriately designed implantationguide device. The index marks then can be visualized during surgery at asubsequent time when another head frame is put on, or no frame is put onat all. The index marks may be visualized by external apparatus, such alinear accelerator collimator system or a fluoroscopic X-ray machine orsome other surgical device. The position of the external apparatus maybe thusly determined relative to the body. CT scanning can be donesubsequently, and the CT data can be related to that Cartesiancoordinate system. This is yet another embodiment of placing indexmarkers in the head or the body so as to relate CT, X-ray, and otherimage modalities one to the other.

DESCRIPTION OF THE FIGURES

The drawings, which are part of this invention specification, showillustrative embodiments demonstrating the objectives and features ofthis invention, and are briefly described as follows:

FIG. 1 is a schematic diagram of a CT system incorporating fiducialpoints on the patient's body in accordance with the present inventionand illustrates the image scanner coordinate system;

FIG. 2 illustrates a head frame or patient attachment means with itsrespective frame coordinate system;

FIG. 3 illustrates an X-ray imaging method in accordance with thepresent invention system which is used to relate the scanner coordinatesystem to the head frame coordinate system by a transformation.

FIG. 4 illustrates the registration of an external apparatus to thepatient's head by means of pre-placed index marks.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

A detailed illustration of several embodiments of the present inventionis described in this specification. It is understood that otherembodiments may be constructed with various forms, some which may differfrom the illustrative embodiments described and disclosed herein. Thus,the present examples by way of figures and this specification are deemedto be desirable embodiments for the purposes of this patent disclosureand to provide examples on which the claims are based to define thescope of the invention, however, it is understood that they are merelyrepresentative and that other forms are possible, based on the presentinvention, by those skilled in the art.

Referring to FIG. 1, the patient 1 is being scanned by a CT scanner inthis figure, which could be by X-rays, NMR (nuclear magnetic resonance),P.E.T. (proton emission tomography), or other computer tomographicmodality. The stack of CT image planes 2 is meant to illustrate severalof the many planes in which the image could be reconstructed. In thisFIG. 1, the image planes are shown perpendicular to the Z axis, whichcharacteristically is the axis of X-ray CT scanners; i.e. perpendicularto the X-ray source gantry. The slices are shown as Z1, Z2, . . . ZN,and represent a stack of reconstructed slices in the axial direction forthis example. The slices could well have been stacked in the coronal orsagittal planes, which are perpendicular to these axial planes. Acoordinate system represented by the axes X, Y, and Z in FIG. 1 would bethe natural coordinate system of the CT scanner itself. The Z axis isperpendicular to the plane of the image slices, and the X and Y axesrepresent respectively the lateral axis and the anterior-posterior (A-P)axes relative to the CT scanning machine. In the typical X-ray CTscanner, the patient would be actually lying horizontally on a tablewhich advances into the aperture of the scanner gantry. The Z axis,referred to as the axial direction, is roughly horizontal andperpendicular to the opening of the gantry aperture. It also runsroughly along the longitudinal axis of the patient's body. The X axiswould actually be perpendicular to the Z axis, and runs roughly from theleft to right orientation of the patient's body. The Y axis is an axisperpendicular to both the Z and X axes, and runs roughly anterior toposterior relative to the patient's body. There is an origin, OI, forthe image scanner coordinate system represented in FIG. 1, which isdefined by the scanner manufacture, and usually includes what would bedefined as the zero slice for which the millimeter index for the slicereads zero on the CT scanner. Thus, each reconstructed planar slicedefines the Z axis, or axial axis, relative to the scanning machine, andeach two-dimensional image produced by such a CT scanning machine willhave its own X and Y axes and be indexed by a Z coordinate value. Thesethree axes are referred to as the scanner axes, and are represented byX, Y, Z in this specification and in FIG. 1. As illustrated further inFIG. 1, the CT image scanner comprises the physical machine 3, whetherit be an X-ray, MRI, or P.E.T. scanning machine or other type ofmodality that determines these tomographic image slices. The data isprocessed by a processor 4 to produce slice displays illustrated by 5 inFIG. 1. These are usually brought up on a display console so that theoperator may examine the anatomy and evaluate normal and pathologicalstructures therein. The data from such a scanning machine may also beseparately taken from the machine and put into a 3-D display processingcomputer illustrated by 6, which will then display in athree-dimensional format on a monitor 7 a representation of thepatient's anatomy. Intrinsic to this data would be the machine-definedcoordinates X, Y, Z, as illustrated in FIG. 1; that is, each point ineach slice, and thus each point or voxel in the 3-D data set, would betagged by its scanner coordinates X, Y, and Z. We have also shown inFIG. 1 index markers 10, 11, and 12, which have been attached to thepatient's physical anatomy. These index markers can be of various typesand forms. They may be radiopaque and/or MRI-visible structures that arestuck to the patient's skin. They may be radiopaque markers that areimplanted in the patient's skull, a more permanent way of placing CTmarkers. They may alternatively be natural landmarks, such as the nasus,auditory meatus, the tip of the nose, or other fairly well definedpoints, lines, or surfaces on the patient's anatomy. They mayalternatively be lines, strips, grids, structures with straight rodscombined with diagonal rods, or other surface structures that could beattached to the patient's anatomy. These markers are also present inpresent or reconstructible in the display 7, and are illustrated here as10A, 10B, and 10C.

An exemplary embodiment for the CT visible markers 10, 11, and 12 wouldbe implanted screws or buttons that can be placed into holes in thepatient's skull so that they will remain with the patient and areabsolutely immovable relative to the skull. The numbers of these markersmay vary depending on the analysis to be described below, but a numbersuch as three or more is desirable. The markers for X-ray CT scanningcould be made from stainless steel, tantalum, or other high Z material.For MRI scanning, they could be capsules with saline solution in them,for example, or they could have a slight magnetization so that on an MRIslice one could visualize a small void where the markers are. Themarkers could also be placed externally on the skin and could berelocated there by small skin tattoos. The markers could be such thatthey are visible in various type of CT scanners (viz., X-ray, MRI, PET,ultrasound, etc.) so that correlative or comparative image orregistration analyses can be done. This is less well defined thanskull-anchored markers since the skin can move, and, after long periods,it may be difficult to identify tattoos which may be near or under thehairline. The markers may further be linear structures or grids,however, point-like markers have an advantage of simplicity.

The purpose of the markers will be to relate the CT image scannercoordinate system to a second coordinate system relative to an attachedhead ring or patient fixation means attached to the body. Each of themarkers is presumed to appear on a CT slice. As illustrated, the imagepoint 15 in slice image 5 in FIG. 1 might represent one of the markers10, 11, and 12 in the actual physical anatomy. By observing thepositions of these image points on the individual 2-dimensional slice aswell as the slice number in which they appear, one can relate thephysical marker positions to the coordinate representation in thescanner coordinate system represented by the axes X, Y, and Z. Modern CTscanners in fact provide the user with an X and Y coordinate position(usually in millimeters) of each point (pixel) in the 2-dimensionaldisplay of a slice, and also readout digitally the slice position(usually in millimeters) corresponding to the position of the CT couchin the gantry, this latter position serving as the Z-coordinate. Thusfor one on a multitude of slices (i.e. a scan volume), the X, Y, Zcoordinates of the index marker and, indeed, all imaged points, isprovided directly from the scanner. This data can be downloaded to otherdevices or computers by standard scanner outputs.

FIG. 1 also illustrates for completeness the transfer from theprocessing elements 3, 4, and 5 (which are usually built into modern CTscanners) to a 3-D display device 6 which can display the volume of CTslice data on display 7. The 3-D processing is not usually part oftoday's CT scanners, but is done by many companies as an ancillaryvisualization means.

Referring to FIG. 2, a second coordinate system, referred to as SF, maybe defined relative to a head ring 220, and is indicated here as X′, Y′,Z′ axes. The origin of the coordinate system is indicated by OF′. Thehead ring 220 is attached to the patient typically by head posts 221 andskull fixation screws 222. The head ring shown in FIG. 2 is similar tothat for the BRW Stereotactic System referred to above. Alternatively,the patient attachment means could be an operating surgical head clamp,which is used for most neurosurgical procedures. An example of such ahead clamp is the Mayfield Head Rest Clamp. It also is attached to thepatient by means of three or more skull screws attached to a semi ringsimilar to that of 220 in FIG. 2. The markers 210, 211, and 212 are alsoshown in FIG. 2 still attached to the patient's head in the sameposition that they were during the CT scan as shown in FIG. 1. Thesemarkers have a known relationship to the image coordinate system SI inFIG. 1, but as yet their position relative to the frame coordinatesystem SF and its respective axes X′, Y′, and Z′, is not known. If theorientation of the markers 210, 211, and 212 can be determined relativeto the frame coordinate system SF, then the relationship of the dataset, as displayed in the three-dimensional representation 7 in FIG. 1can also be related quantitatively to the frame coordinate system.

One embodiment of the present invention to make such a transformationbetween image coordinate system and frame coordinate system isillustrated in FIG. 3. Here two X-ray plane films are taken of thepatient with the head ring 320 attached to his head (the head posts arenot shown, but are assumed to be there in this picture). Shown in FIG. 3are two X-ray film planes 330 and 331 and two corresponding sources ofX-rays 323 and 324, respectively. The first source is a lateral X-raysource 323, and the other is an anterior-posterior (A-P) source 324.Attached to the head ring 320 are radiopaque square structures 325, 326,327, and 328, which will appear on the X-ray films as squares,rectangles, or generally quadrilaterals, depending on the orientation ofsource 323 and film plane 330. Thus, for example, with source 323, whichis located on the right side of the patient as it radiates through thepatient's head, and the square structures 325 and 327, which areattached in a known position relative to head ring 320, the squares 325and 327 will project as quadrilaterals 325A and 327A on the X-ray planeimage 330. Similarly, for the A-P source of X-rays 324, the images ofthe squares 326 and 328 will appear as quadrilateral images 328A and326A on the planar X-ray film plane 331. Also shown in FIG. 3 are theprojected images of the markers 312, 310, and 311, as they appear on theX-ray planes 330 and 331. These are illustrated respectively as 310A,311A, and 312A, and on the plane 330, and on the plane 331, they are310B, 311B, and 312B. It is well known, and has been implemented forsome years, that such lateral and frontal X-ray views with indexsquares, such as 325, 326, 327, and 328, projected on them will enablethat the quantitative position of well identified objects, such as themarkers 310, 311, and 312, when seen in both such planar X-ray images,can be mathematically determined relative to the head ring 320. Thismeans that by quantitative analysis of the images on each plane 330 and331 the actual coordinate values of each of the markers, i.e., the X′,Y′, Z′ coordinates relative to the frame coordinates SF can bedetermined for each one of these markers. For a discussion of how thisis done, the paper of Vandermeulen, et al. illustrates the technique,and the commercially sold product by Radionics, Inc., referred to as theSGV-AL Angiographic Localizer does precisely this operation. Therefore,from such a configuration and method as illustrated in FIG. 3, the framecoordinate values for the markers 310, 311, and 312 can be definitivelydetermined with great accuracy. From such a calculation, the X′, Y′, andZ′ coordinate values for each of the index markers 310, 311, and 312 canbe calculated. The image processing of the data from FIG. 3 and thesecalculations might be done by the module 341 illustrated schematicallyin FIG. 3.

As has been described above, the X, Y, and Z coordinates for each of theindex markers have also been determined in the scanner coordinatesystem. This means that each of the markers has been assigned a value X,Y, and Z by this means. The X and Y values from the image scanner aredisplayed on its two-dimensional readout, as shown in FIG. 1. The Zcoordinate may be determined typically by knowing which slice number theindex marker appears in. Typical CT scanners have metric readouts of theslice position with considerable accuracy, and thus the calibration in Zin the scanner coordinates may be determined directly from the relativeposition of the scanner couch and the scanner gantry with theappropriately built-in readout means. Thus, in fact, every visualizedpoint of the image scanner can be assigned an X, Y, and Z coordinatevalue, not only the markers themselves. Thus, once the determination hasbeen made of the linear transformation which maps each point X′, Y′, andZ′ of the imaging coordinate system to each point X′, Y′, and Z′ of theframe coordinate system, then the full correspondence between each pointseen in the image scan data can be made relative to the frame coordinatesystem. Thus, the data from the imaging system can be transposed intodata which corresponds to the frame coordinate system. That coordinatetransformation can be made in several ways. One method illustrated hereis that by knowing the individual coordinates of each of threeindependent markers, such as 310, 311, and 312, in FIG. 3 for the imagecoordinate system and the frame coordinate system, then thetransformation matrix for these marker points can be determined, andthus the full transformation between any coordinates in the imagecoordinate system can be made to the frame coordinate system and viceversa.

A more specific illustration of the calculations discussed will now begiven. To make the transformation alluded to in FIG. 3, the head ring320 has affixed to it four plates, referred to as 325, 326, 327, and328. Each of these plates has a square pattern on them, either asradiopaque lines or radiopaque dots indicating the corners. This is thegeometry of the commercially available SGV-AL Angiographic Localizerfrom Radionics, Inc. When the lateral and the A-P X-ray is taken throughthe angiographic localizer and also the patient's head by means of theX-ray sources 323 and 324 respectively, the square patterns will show upas quadrilateral image patterns on the plane film X-ray views 331 and330 respectively. These patterns are shown in FIG. 3 as thequadrilateral image lines 325A, 327A, 326A, and 328A. In addition, oneach of the images is seen the patient's anatomy, as well as the imagesof the index markers 310, 311, and 312. These images of the imagemarkers appear on the lateral X-ray image as 310A, 311A, and 312A, andon the A-P image as 310B, 311B, and 312B. Thus, each of the X-ray imageshas fiducial markings associated with the stereotactic apparatus, aswell as fiducial markers representative of the position of the patient'sanatomy. As mentioned above, the fiducial markers relative to thepatient's anatomy are known in the scanner coordinate system since theyappear on the scan images, and are thus referenced relative to the scancoordinate system. The object of the instrumentation in FIG. 3 is todetermine the coordinates of the reference markers in the stereotacticcoordinate system X′, Y′, and Z′ as shown in FIG. 2. This can be done bya method of projective geometry, which has been described in the paperof Vandermeulen, et al., referred to above. The process for making suchcalculations in frame coordinates is handled by the image acquisitionelements 340 in FIG. 3, which involve either digitization of the entireX-ray images or will involve point-by-point pickoff of the coordinatesof all of the structures in each of the lateral and A-P images.Information from the data acquisition is then fed into a data processingand stereotactic frame coordinate calculation instrumentation indicatedby 341 in FIG. 3. From this the X′, Y′, and Z′ coordinates for each ofthe index markers 310, 311, and 312 relative to the stereotactic framecan be calculated uniquely.

At this point, the coordinates for the three markers are known in boththe image coordinate system and the frame coordinate system, and alinear transformation F can be carried out to relate thereby any pointin the image coordinate system SI to their associated position orcoordinates in the frame coordinate system SF. [F] is a 3×3 matrix withcoefficients F_(nm). Let the 3 markers be designated as A, B, and C. ForA, its coordinates in SF are A′_(n), where n=x′, y′, or z′; and itscoordinates in SI are A_(m), where m=x,y, or z indices. Then

A′ _(n) =F _(nx) A _(x) +F _(ny) A _(y) +F _(nz) A _(z), for n=x′,y′,andz′.

Similar equations hold for B′_(n) and C′_(n). From these 9 equations,the coefficients of Fnm can be determined, since all the coordinates ofA, B, and C are known. Hence, for any other point D in the space,

D′=[F]D

i.e. the transformation from SI to SF is known. These matrix methods arestandard in linear algebra. In addition, one can add to thesetranslations a linear offset or translation term to take account ofdifferent origins of the transformed coordinate systems.

Once such a transformation has been made, then some or all of the imagedata of the patient's anatomy, as seen on the CT scanner, can betransformed into frame coordinates and can be represented inthree-dimensional space for visualization and analysis, as representedby the process of 343 and the image display means 344 in FIG. 3.

It is understood that variations in the specific nature, form, andprocess of the embodiments shown in FIG. 1, 2, and 3 are possible. Suchvariations where they do not depart from the overall concept of thepresent invention we will claim accordingly in the claims of this patentapplication below. Examples of such variations would be differentconfirmations of head rings, as shown in FIG. 2, variations in theconcept of index markers. For example, instead of point or point-likeindex markers, the index markers could be lines, grids, volumes, or havedistinguishing shapes or signature so that they can be identified onefrom the other. Markers can be made out of a variety of materials,including metals, composites, radiopaque or MRI illuminating filledmedia, vessels that can contain fluid for MRI visibility or P.E.T.scanning, screws that can be anchored to the skull, patches that can bestuck to the skin, wires that can be laid on the surface of the anatomyor embedded under the skin, and other such variations. The angiographicor X-ray squares, as shown in FIG. 3, can also be varied in othergeometries or configurations so as to index the planar views 330 and 331to make the coordinate transformation. Natural anatomical landmarks canalso be used for this purpose rather than index markers placed in theanatomy itself. For example, the point of the nose, the nasium, thepoints or positions of the ears, etc. could be used. Tattoo marks couldbe placed on the skin, and the markers could be placed on and taken awayat the time or after imaging or surgery. To achieve the images in FIG.3, simple two-directional X-ray machines may be used, or fluoroscopy maybe used. Such machines are readily available in operating rooms, so thiscan be done at the time of surgery. In the case of radiosurgery orradiotherapy, such X-ray shots can be taken as the patient is placed onthe radiation machine, and thus the transformation from imagecoordinates to frame coordinates can be done immediately or used as averification check. Different types of secondary imaging modalities maybe used such as ultrasonic rather than X-rays.

In the situation of surgery, very frequently a head holder or head clampin attached to the patient's head, or other anatomy, after he isanesthetized. Then a sterile draping is placed over the head and thehead clamp or head ring in preparation for surgery. During surgery, astereotactic arc or an operating arm may be used to identify anatomicalpoints in the surgical field, and these points should be referenced tothe CT scanner information. The head ring may have adaption means sothat the angiographic or X-ray plates, such as in FIG. 3, can beattached to the ring through the sterile drape at the time for surgery,and the coordinate transformation confirmed or calculated during surgeryto assure that the surgeon has the proper orientation of the anatomyread relative to the head ring. Thus, subsequently, when a stereotacticarc or an operating arm is attached to or related to the position of thehead ring 320 in FIG. 3, the corresponding associated relationships tothe patient's anatomy will also be known. This is assured by taking suchX-ray films as shown in FIG. 3 and doing the transformation and thuscoordinate system mapping, as described above. In the field of“frameless stereotaxy” where a scan is done without a head holder on thepatient's head, and later in a surgical setting the head clamp or headring is placed on after anesthesia, such a check or coordinatetransformation at the site of surgery is very important. Use of discreteindex markers, such as small screws implanted in the skull, give a levelof assurance in this operation. Such screws would not move and would becarried with the patient indefinitely. Thus, in a situation of repeatsurgery or when the surgery is separated widely from the time of CTscanning, one is assured that the index marks have not moved, and thustheir transformation, as illustrated in FIG. 3, will provide a faithfulmapping from CT image coordinates to the apparatus associated with thesurgery or radiation therapy.

Other forms of index imaging, such as illustrated in FIG. 3, may also bedevised. Instead of two planar X-ray films, a CT or MRI scan may beperformed after the initial scan, and the positions of the index markersseen again on another of these tomographic type processes. By so doing,again the position of the index markers can be transferred from onecoordinate system to another. Accordingly, FIG. 3 is meant only as anone example of the secondary imaging process in which the index markersare seen again and their position calculated relative to the secondaryimaging process apparatus, and thus the transformation made to theoriginal CT scanning image in which the markers are seen first.

In addition to the examples given above, other implementations of thepresent invention can be thought of. One such implementation isillustrated in FIG. 4. There the index marks 410, 411, 412, and 414 areplaced on an orthogonal coordinate system or a Cartesian coordinatesystem in the patient's head. Another index spot, 415, might be put onthe node or zenith of such a coordinate system. This can be implementedby a special guidance apparatus that could be devised so as to insertthe index marks on such a coordinate or Cartesian axis. It is notnecessary that all index marks are on such a Cartesian axis, but thiscould be convenient in some situations. Subsequently, a head ring, asillustrated by 416, may or may not be placed on the patient's head. Ifsuch a ring is placed on the patient's head, then external apparatusindicated schematically by the arm 402 may be known as orientationrelative to the head ring 416. If one wishes to know the orientation ofthe apparatus 402, which is simply diagramized as a bar in this case,but could be a much more complicated stereotactic or guidance means,then one could use an X-ray tube or X-ray beam in a guidance device suchas 401 to orient the apparatus along the line between index points 410and 412. This could be determined by means of the X-ray screen 403 andthe associated dots 414A and 412A, which will be images of the indexmarks 412 and 414 respectively on the screen. Thus, by orienting theX-ray source and thus the guidance apparatus 401 and 402 so that theimage spots are coincident on the screen 403, one automatically lines upthe external apparatus 402 relative to that axis in the patient's body.Similar steps could be taken to align an apparatus along the lines ofindex points 411 and 414. In this way, one can establish with externalapparatus the same coordinate system that was determined previously bythe implantation of the index marks.

Subsequent CT imaging could then visualize these marks or dots and theirreference to the entire anatomy as seen on the CT or MRI image data. TheCT image data coordinate frame can be related to the index mark framevia a transformation or a simple stacking of data which is in planesparallel to the principal axes of the Cartesian coordinate systemrepresented by the index marks. Thus one has the ability with such indexdots to alternate between image modalities and thus transform orreplicate orientations from one episode to the next.

One can also align the X-ray source or fluoroscopic C-Arm 401 so that itmerely picks up the images of 410 and 412 on a screen 403. The relativeorientation of those images could determine the position of the X-raytube 401 relative to the anatomy. In other words, a coincidence of theimages 412A and 414A is not necessary to establish the orientation. Thisis similar to what was illustrated in FIG. 3 previously. Index reticulesor bomb sites could be attached to the head ring 416 so as to putfurther reference lines or marks on the screen 403 with the index images412A and 414A so as to produce further reference information.

It is to be recognized that this technique can be used throughout thebody, and the example in the anatomy of the head is used only asillustration. It is also recognized that it can be used in conjunctionwith a variety of other processes, methods, or surgery involvingstereotactic arcs, operating arms, robotic devices, and invasive ornon-invasive probe or diagnostic means. Various forms of the CT scanner,display devices, and data processing components may also be envisaged,and it is intended that this invention claims such variations of method.Now, having described the invention, the scope shall be definedgenerally by the claim set forth below:

What is claimed is:
 1. A method of relating the orientation of anexternal apparatus relative to patient's anatomy as determined fromcomputer tomographic (CT) image data, including: (a) attaching to saidpatient's anatomy detectable markers which are detectable in said CTimage data to establish a coordinate reference frame relative to saidpatient's anatomy, so that said detectable marker positions can bedetermined in said CT image data of said patient's anatomy; (b) takingimages from at least one direction with a secondary imaging means, theorientation of said secondary imaging means being determinable relativeto external apparatus, so as to detect said detectable markers andthereby determine the orientation of said patient's anatomy asvisualized in said CT image data relative to said external apparatusthat is in a determined orientation relative to said secondary imagingmeans.
 2. In connection with computer tomographic (CT) scanner and X-rayimaging apparatus, a system for relating external apparatus to patient'sanatomy as visualized from said CT scanner image data including: (a) CTimage detectable markers that can be attached to said patient's anatomyand which can be visualized and thus related spatially in said CTscanner image data relative to said patient's anatomy; (b) externalapparatus which can be located in a stable position near said patient'sanatomy, said external apparatus including reference structures attachedto said external apparatus which can be seen in X-ray images from saidX-ray imaging apparatus of said patient's anatomy and said externalapparatus, whereby upon X-ray imaging of said patient with said externalapparatus nearby said patient, said reference structures and saiddetectable markers can be seen in said X-ray imaging, and thus theposition of said external apparatus can be determined relative to saidpatient's anatomy.
 3. In connection with computer tomographic (CT)scanner and a secondary external imaging device, apparatus to relate theimage data of said patient's anatomy from said CT scanner to that ofimage data from said external imaging device, and thus to support meansthat is in a known orientation relative to said external imaging deviceand to said patient's anatomy comprising: (a) index means that can befixed to said patient's anatomy and that are detectable in said CTscanner image data and also are detectable in said external imagingdevice data, and thus the position of said index means relative to saidpatient's anatomy can be determined in said CT scanner image data andsaid external imaging data; (b) support means adapted to be held in astable position relative to said patient's anatomy and adapted so thatthe position of said support means can be determined relative to saidexternal imaging device; whereby when said support means is positionedrelative to said patient's anatomy and said external imaging device ispositioned relative to said support means, image data from said externalimaging device of said patient's anatomy and of said index means can beused to determine the orientation of said support means to saidpatient's anatomy as visualized by said CT scanner image data.
 4. Theapparatus of claim 3 wherein said support means is a rigid structurewith radiopaque index means attached to it, and said external imagedevice is an X-ray machine.
 5. The apparatus of claim 4 wherein saidindex means are radiopaque point-like structures located in knownpositions relative to said support means.
 6. The apparatus of claim 5wherein said index means are radiopaque marks that can be attached in afixed position relative to the bony structures of said patient'sanatomy.
 7. The apparatus of claim 3 wherein said index means aredetectable in an X-ray CT scanner or on magnetic resonance CT scanner sothat comparative images from said scanners can be utilized.
 8. Themethod of claim 1 wherein said secondary imaging means comprises acomputer tomographic imager.
 9. The method of claim 1 wherein saidsecondary imaging means comprises an X-ray imager.
 10. The method ofclaim 1 wherein said secondary imaging means comprises a magneticresonance imager.
 11. The apparatus of claim 5 wherein said index meanscomprises radiopaque indices.